The photographic technique in which silver halides are used is superior the electrophotographic and diazophotographic techniques with respect to photographic characteristics such as photographic speed and gradation control. Accordingly, in the past this has been the most widely used technique.
These silver halide photographic techniques include wet processing type color diffusion transfer techniques in which a dye fixing element which has a dye fixing layer is laminated with a photosensitive element which has a silver halide emulsion layer Further, an alkaline processing composition is spread in the form of a layer within this laminate or the laminate is immersed in an alkaline processing fluid.
Furthermore, techniques in which diffusible dyes are produced or released in correspondence or in counter-correspondence to the reaction when a photosensitive silver halide and/or organic silver salt is reduced to silver by thermal development and in which the diffusible dyes are transferred to a dye fixing element have been developed more recently. Such techniques have been disclosed, for example, in U.S. Pat. Nos. 4,463,079, 4,474,867, 4,478,927, 4,507,380, 4,500,626 and 4,483,914, JP-A-58-149046, JP-A-58-149047, JP-A-59-152440, JP-A-59-154445, JP-A-59-165054, JP-A-59-180548, JP-A-59-168439, JP-A-59-174832, JP-A-59-174833, JP-A-59-174834, JP-A-59-174835, JP-A-62-65038, JP-A-61-23245, and European Patents laid open 210,660A2 and 220,746A2. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".)
However, with such image forming techniques some dye transfer occurs when, after image formation, a dye fixing element is stored on top of another dye fixing element under conditions of high humidity and there is sometimes a loss of image density. Problems can also arise with so-called color transfer where the white base of the dye fixing element becomes colored. Moreover, with the above mentioned method of image formation, the dye fixing element is likely to curl, especially under conditions of low humidity. There are also problems with uneven transfer due to the fact that the contact with the photosensitive element at the time of image formation is not made satisfactorily, and with the possible occurrence of transportation failure in the image formation processing apparatus.
Moreover, when image formation is carried out after ageing and storage with the method of image formation described above, pronounced staining of the white base may occur and there is the further problem that color impurity may arise.
Furthermore, the use of polysaccharides such as dextran and pluran in addition to gelatin or in place of gelatin as the binder for the constituting layers of diffusion transfer photographic photosensitive materials has been disclosed in JP-A-62-47636. However, coated layers which contain dextran and pluran cannot be used in drying methods in which gelation is employed, because of the problem of drying attachment.
Heat developable photosensitive elements and the processes involved therein which are known in this field of technology have been described, for example, in The Basics of Photographic Optics, Non-silver salt photography edition, pages 242-255 (published by Corona, 1982), and in U.S. Pat. No. 4,500,626.
Many methods have been proposed for obtaining positive color images by means of thermal development.
For example, a method has been proposed in U.S. Pat. No. 4,559,290 a compound sometimes using known as a DDR compound in its oxidized form (which has no dye releasing capacity) and a reducing agent or a precursor thereof. The reducing agent is oxidized in accordance with the exposure of the silver halide by thermal development. The compound is then reduced by the remaining reducing agent which has not been oxidized and releases a diffusible dye. Furthermore, heat developable color photosensitive elements in which non-diffusible compounds which release diffusible dyes as a result of the reductive cleavage of an N--X bond (where X represents an oxygen, nitrogen or sulfur atom) are used as compounds which release diffusible dyes via a similar mechanism have been disclosed in European Patent Laid Open 220,746 and in Kokai Giho 87-6199 (Volume 12, No. 22).
Electron donors and electron transfer agents are generally used conjointly as the reducing agent in the methods of positive image formation in which reducible dye donating compounds of the type described above are employed.
However, with methods of image formation wherein an electron transfer agent is combined with an electron donor of this type and heat development is carried out, the capacity for reducing silver halide to silver is increased and there is the problem that the maximum density of the positive image is reduced because this reduction reaction occurs even in the unexposed parts.
Furthermore, various improvements have been made with the coating apparatus, the method of drying and the coating liquid formulation to ensure that coating liquids are coated uniformly on the support.
In these improvements, use is generally made of a water soluble binder which undergoes a reversible sol/gel transformation when the aqueous solution is heated and cooled. Methods in which the coated layer is established on a support are generally used in the photographic industry for example.
However, uneven coating is likely to arise as a result of the drying in the case of dilute aqueous solutions of the above mentioned water soluble polymers and in cases wherein a substance which inhibits gelation is added to the coating liquid. Furthermore, unevenness in the thickness of the coated layer arises as a result of the phenomenon known as ribbing when a coating step is carried out on a support which has a certain degree of roughness.
The occurrence of such uneven coated layers and uneven thickness results in a reduction of the quality of the intended product. In particular, when the intended product is a photographic element (a photosensitive element or an image receiving element), there is the problem that photographic quality is not constant For example, if such an unevenness of coating or unevenness of coated thickness arises in the image receiving layer (known as the dye fixing layer) which is used for forming an image by the transfer of image forming components from a photosensitive element, the diffusion distance of the image forming components will differ somewhat depending on the location. Further, there may be parts where there is no contact between the photosensitive element and the image receiving material Unevenness will therefore arise in the image which is formed and only a low quality image will be obtained.
In particular, in cases where natural macromolecular polysaccharides (for example, natural macromolecular polysaccharides originating from red algae such as phaeceleran and carrageenans for example, and natural macromolecular polysaccharides obtained by fermentation such as "Jelan gum" for example) are used as water soluble polymers which undergo reversible sol/gel transformation when an aqueous solution is heated and cooled, the powders are difficult to dissolve in water. Undissolved lumps known as "dama" or "mamako" are likely to be formed and they are likely to result in unevenness on coating.